1. Field of the Invention
The present invention relates to a semiconductor light-receiving device and a manufacturing method thereof, and particularly to a technique for suppressing dark current and deterioration.
2. Description of the Background Art
Conventionally, mesa-shaped semiconductor right-receiving devices have been in frequent use because they are easy to manufacture.
For example, in a conventional mesa photodiode (hereinafter “photodiode” will be referred to as PD), the mesa structure is formed by removing the part around the light-receiving portion, from a p-type semiconductor layer on the surface side to an n-type semiconductor layer on the substrate side, including an i-type light absorption layer. Furthermore, with this structure as a first mesa, a second mesa is formed by removing part of the material such that the mesa diameter of the p-type semiconductor layer and the mesa diameter of an upper part of the light absorption layer are smaller than the mesa diameter of the main part of the light absorption layer. By this, the depletion region in the light absorption layer is not exposed in the first mesa surface, and dark current and device capacitance are reduced (for example, see Japanese Patent Application Laid-Open No. 4-332178 (1992), which is hereinafter referred to as Patent Document 1). For such a PD, it is clear that a similar operation is achieved when the p and n conductivity types are used in a reverse manner.
Also, in a mesa avalanche photodiode (hereinafter “avalanche photodiode” will be referred to as APD), the area around the light-receiving portion is removed with a slope, including an n-type semiconductor layer on the surface side, a light absorption layer, and a pn junction surface of an avalanche multiplication layer under the light absorption layer, resulting in a sloped “normal” mesa structure in which the avalanche multiplication layer is larger than the light absorption layer (for example, see Hiroo Yonezu, “Optical Communication Device Engineering (Hikari Thuushin Soshi Kougaku)”, Kougaku Tosho, p. 398 (FIGS. 7, 6), p. 419 (FIGS. 7, 18), 1984 (S. 59), which is hereinafter referred to as Non-Patent Document 1). With such an APD, it is clear that dark current and device capacitance can be reduced by forming a second mesa so that the mesa diameter of the n-type semiconductor layer is smaller than the mesa diameter of the light absorption layer, in a way similar to the formation of the PD of Patent Document 1. It is also clear that a similar operation is achieved when the p and n conductivity types are used in a reverse manner. When the avalanche multiplication layer is provided above the light absorption layer, the APD is shaped in a sloped “reverse” mesa so that the avalanche multiplication layer is larger, because it is preferable that the ionization rate of the carriers, i.e. electrons or holes, injected from the light absorption layer be higher than that of the other.
Also, planar light-receiving devices are in frequent use because they are highly stable. For example, in a conventional pseudo planar light-receiving device, the planar structure is achieved by forming a trench to isolate the light-receiving portion of a p-type semiconductor layer on the surface side, or by removing the part around the light-receiving portion (for example, see International Publication WO 2006/123410, which is hereinafter referred to as Patent Document 2).
With such semiconductor light-receiving devices, during the process of semiconductor crystal growth, the dopant used to give conductivity to the conductive semiconductor layer on the surface side thermally diffuses into the light absorption layer, and thus gives conductivity also to the light absorption layer. The presence of the conductive region diffused into the light absorption layer causes expansion of the depletion region occurring in the light absorption layer, and it is therefore necessary to remove the conductive region diffused into the light absorption layer so that the depletion region will not be exposed in the first mesa surface at all. As a result, the depletion region is exposed in the side surface of the second mesa, resulting in increased dark current.
Furthermore, the depletion region is exposed in the first mesa side surface and the second mesa top surface in the light absorption layer that has a small band gap, and therefore the device deteriorates from the exposed portion and fails to offer reliability. In particular, the deterioration from the mesa interface is especially serious in the case of APDs that involve generation of high electric fields.